1. Field of the Invention
The present invention relates to an origin detection method for an optical encoder that is used in displace measurement or angle measurement.
2. Description of the Related Art
A photoelectric encoder has a main scale on which a first optical grating is provided and an index scale opposed thereto on which a second optical grating is provided. The photoelectric encoder further has a light source that illuminates the main scale with light and a light receiving element that receives light having been transmitted through or reflected by the optical grating of the main scale and transmitted through the optical grating of the index scale.
Japanese Patent Publication No. H06-056304 teaches use of a light receiving element array that also functions as an index scale in a photoelectric encoder of the above described type. The inventors of the present invention have also filed patent applications, e.g. Japanese Patent Application Laid-Open No. 2003-161645, on inventions concerning encoders of the above described type.
The encoder having the above described structure is called an incremental type encoder. This type of encoder detects the movement amount of a scale by counting the number of output pulses generated by movement of the scale. A problem encountered with the incremental type encoder is that the absolute position in the rotational angle cannot be determined, and it is required, in some cases, to provide an additional separate sensor to detect the absolute position.
As a solution to this problem, the following system has been developed. FIG. 16 shows a scale disclosed in Japanese Patent Application Laid-Open No. H10-318790, in which the transmittance of the pattern on the scale 1 is varied to enable to detect the absolute position in an incremental type encoder. In this scale, mark 1a has a transmittance of 1, and transmittances of marks 1b, 1c, 1d . . . gradually decrease.
FIG. 17 shows changes in signals that occur when a portion of the scale 1 in which the transmittance varies passes by a sensor in an encoder having this scale 1. Signals A and B are analogue two phase signals obtained from the sensor.
The amplitude of the signals decreases with a gradual decrease in the transmittance of the mark of the scale 1, and the absolute position can be detected by detecting this change in the signal amplitude.
In an absolute position detection unit used in the above described conventional encoder, in order to detect the signal amplitude, it is required to sample signals at intervals significantly shorter than a cycle of the encoder signals obtained.
Since it is necessary to determine the peak voltage and valley voltage of the signals based on the result of sampling, a large scale circuit such as a high speed A/D converter is required to be provided.
In addition, it is difficult to produce a scale including portions having different transmittances with reliability, and significant variations in actual changes in the signal amplitude will be generated.
An object of the present invention is to provide an origin detection method for an optical encoder that enables origin detection with a simple structure by performing computation according to changes in the sensor signal with movement of a scale.